The present invention relates to apparatus and a method for sensing or measuring magnetic fields and hence also to apparatus and a method for sensing or measuring electric current. Temperature may also be measured. A particular application is in measuring high fault currents in electrical power and distribution systems. The invention could, for example, be used in mapping magnetic fields around high power transmitters or for condition monitoring in electrical machinery.
Considerable research has been carried out into the use of the Faraday effect, which is described below, in measuring large electrical currents. Measurement depends on the Verdet constant V(.lambda.,T) which is both dispersive and temperature dependent. Most techniques have concentrated on single mode optical fiber as the sensing element because of its well-known desirable properties; electrical isolation, flexibility, linear response, large bandwidth and zero hysteresis. However, optical fibers still suffer from several serious problems. Firstly, environmental birefringence perturbations, the most serious of which arises from vibration, which causes changes in the state of polarization (SOP) of the optical beam with concomitant sensitivity and scale factor fluctuations. Various techniques have been investigated to reduce this effect including the use of very low intrinsic birefringent fiber, the development of spun high birefringence fiber, the use of jelly filled cabling to mechanically isolate the optical fiber and the deployment of low birefringence fiber in a helical configuration to take advantage of geometrical birefringence effects. These techniques have met with varying degrees of success, but all suffer from varying problems. A major problem is the low Verdet constant of silica fibers (.about.5.times.10.sup.-6 rad A.sup.-1 ) which then requires many turns of fiber to increase the sensitivity. However, this then leads to greater birefringence in the fiber introduced when the fiber is deployed in a loop around the current carrying conductor, and hence larger diameter loops are required; the birefringence is proportional to (1/R.sup.2) where R is the loop radius. In addition this technique cannot be used for small sensing elements, for example localized sensors for mapping magnetic fields around high power transmitters or for condition monitoring in electrical machinery. Optical fiber has recently been employed as the sensing element in constructing an electric current sensor for fault protection, up to 60 KA, on a 145 KV power line. However, the loop diameter is .about.1 m and up to 7 turns of fiber are required to give the required dynamic range. This technique is inappropriate for the smaller sensing configurations mentioned previously, and where lower currents, 1 to 1000A, are required; the reduction in coil diameter combined with the increase in the number of fiber loops required would induce a serious degree of linear birefringence thus degrading the SOP of the light in the sensing element with a concomitant reduction in measurement sensitivity and with increased environmental sensitivity.
Techniques to increase the sensitivity, and thus allow smaller and easier methods to deploy sensors, have been investigated based predominantly on using materials which exhibit higher Verdet constants. Generally, these have been glass blocks made from flint or lead glass (see: T. Yoshino, "Optical fiber sensors for electric industry", SPIE, 586, 30, 1985; and Y. Kuroda, Y. Abe, H. Kuwahara and K. Yoshinaga, "Field test of fiber-optic voltage and current sensors applied to gas insulated substation", SPIE, 586, 30, 1985) with limited investigation of FR-5 glass (See: R. P. Tatam and D. A. Jackson, "Remote probe configuration for Faraday effect megnetometry", Optics commun. 72, 60, 1989). FR-5 has a Verdet constant of .about.10.sup.-4 rad A.sup.-1, typically 20 times greater than silica fiber. It is a glass doped with paramagnetic Nd ions and therefore V(.lambda., T) follows a 1/T dependence; silica based optical fiber and flint glass are diamagnetic materials and consequently the temperature dependence of V(.lambda., T) Is negligible. This temperature dependence of V(.lambda., T) for paramagnetic based sensing elements causes severe problems as changes in the recovered Faraday signal due to temperature changes are indistinguishable from changes in the electric current/magnetic field, i.e. the scale factor is temperature dependent. For example, for an operating temperature range of -20.degree. C. to +60.degree. C. the Verdet constant for FR-5 changes by .about.30%. Fault protection requirements require .about.5% accuracy and metering 0.1 to 1% accuracy and therefore temperature compensation is essential. Previous research has concentrated on signal processing techniques associated with recovering the Faraday signal and the techniques developed have significantly improved methods of deployment, allowing remote operation of sensing elements, as well as overcoming problems associated with `down-lead` sensitivity and operating point drift. However, the temperature sensitivity was not addressed in these investigations. Several techniques for temperature compensation have been proposed based on the sensing element experiencing a permanent d.c. magnetic field component. The major disadvantage with these techniques is the requirement for permanent magnets to be positioned at the sensing element, thus requiring shielding from the very high magnetic fields present. In addition such techniques would be impractical with high Verdet constant optical fiber since large diameter and long length magnets would be required. Another proposed technique uses a bimetallic coil to mechanically rotate the sensing element thus increasing or decreasing the effective magnetic field component acting on the sensing element. This technique requires the sensing element to be mounted in a complex mechanical assembly which is able to rotate over .about.30.degree. and is therefore unattractive for ease of installation and long term reliability. According to a first aspect of the present invention there is provided apparatus for sensing or measuring a magnetic field comprising
a light source for generating light having at least a component which is polarized, PA1 a sensing element arranged to transmit light from the source and comprising material having a Verdet constant which depends on the temperature of the material, PA1 means for combining light from the source with light from the sensing element to form an interference pattern, PA1 means for providing a signal representative of the intensity of light so combined, and PA1 means for deriving first and second signals dependent on the Verdet constant of the material and the temperature thereof, respectively, from the signal representative of light intensity, whereby the intensity of the magnetic field at the element, substantially independent of temperature, can be derived. PA1 generating light having at least one component which polarized, PA1 transmitting the light generated through a sensing element in the magnetic field, the sensing element comprising material having a Verdet constant which depends on the temperature of the material, PA1 combining light from the source with light from the sensing element to form an interference pattern, PA1 providing a signal representative of the intensity of light so combined, and PA1 deriving first and second signals dependent on the Verdet constant of the material and the temperature thereof, respectively, from the signal representative of light intensity, whereby the intensity of the magnetic field at the element, substantially independent of temperature, can be derived.
Preferably the means for combining is a Michelson interferometer and the sensing element is a high Verdet constant material in the form of a glass block or optical fiber, for example. The use of such materials allows a higher sensitivity and smaller size sensing element. However, in some circumstances a specific material, such as a particular optical fiber, may have certain advantages even though it has a relatively low Verdet constant. Such materials may be used according to the invention where the Verdet constant is temperature dependent.
The technique is completely passive and requires only the sensing element to be placed in the measurement field; permanent magnets and mechanical compensation are not required.
The apparatus of the first aspect of the invention may include means for deriving a signal representative of the magnetic field at the element from the first and second signals.
To simplify signal processing the apparatus may include means for modulating, with the said light intensity, a carrier signal such as can be generated, for example, by cyclically varying either the frequency of the light or the length of one of the interferometer arms. Means for demodulating the carrier signal may then be used to derive the first signal which varies according to the Faraday effect and temperature, and the second signal which is dependent on temperature but not the Faraday effect.
The sensing element of the invention is completely passive and optical links between the interferometer and both the light source and an optical sensor, forming part of the means for sensing light intensity and change in phase, are preferably by way of a high birefringence monomode fiber and a multimode fiber, respectively, allowing remote deployment of the sensing element. Environmental perturbation of these fiber links should not give rise to signal degradation. The small size and undistorted nature of its deployment, when not bent to form a loop, should result in significantly smaller susceptibility of the sensing element to environmental perturbations, particularly vibration, that give rise to signal fading and scale factor changes.
According to a second aspect of the invention there is provided a method of sensing or measuring magnetic field intensity comprising the steps of
When polarized light is transmitted by a dielectric element in a magnetic field it experiences Faraday rotation. The Faraday effect is the rotation of the azimuth, .PHI..sub.F, of a plane of polarized light in the presence of a magnetic field component, H, parallel to the direction of propagation of the light and is given by EQU .phi..sub.F =V(.lambda., T) H.dL equation 1
where V (.lambda., T) is the material dependent Verdet constant, which is both dispersive and temperature (T) dependent, .lambda. is the wavelength of the light and L is the interaction length.
If the dielectric element is used as a sensing element and forms part of a Michelson interferometer illuminated by linearly polarized light coupled from the source to the interferometer using high birefringence fiber then conventional Michelson fringes are formed at the output. The transfer function for a Michelson interferometer may be written EQU I.varies.(I.sub.1 +I.sub.2)(1+.nu.cos .DELTA..PHI.) equation 2,
where I is the observed intensity, I.sub.1 and I.sub.2 are the optical intensities in the two paths of the interferometer, .nu. is the visibility and .DELTA..PHI. is the phase difference between the two recombining optical beams.
In equation 2 the visibility .nu. depends on I.sub.1, I.sub.2, the coherence length of the source and the relative state of polarization (SOP) of the two recombining beams. From equation 1 the relative SOP depends on both intensity of the magnetic field and temperature but phase difference, .DELTA..PHI., that is the phase of the output fringes, depends only on temperature. Temperature change in the sensing element alters the optical path length, predominantly by means of a change in the refractive index, and hence varies .DELTA..PHI.. Thus a measurement of this phase change gives direct information on temperature change occurring in the sensing element independent of the applied magnetic field. This temperature information is used to calculate the Verdet constant and allows the Faraday rotation to be used for magnetic field measurement if compensated for temperature changes of the sensing element.